BMI (Body Mass Index) Calculation

BMI is the ratio between your weight and your height squared. It's the most widely used indicator in the world to evaluate body size. Simple, fast, but should be interpreted with perspective.

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How is BMI calculated?

The formula is simple: BMI = weight (kg) ÷ height² (m²).

Example: 75 kg for 1.78 m → 75 ÷ (1.78 × 1.78) = 75 ÷ 3.17 = 23.7.

The World Health Organization classifies results into 4 categories: underweight (< 18.5), normal weight (18.5 – 25), overweight (25 – 30), obese (> 30).

The limits of BMI

BMI doesn't distinguish between muscle and fat. A 100 kg rugby player standing 1.80 m tall will have a BMI of 30.9 ("obese") even though he is in peak physical condition.

BMI doesn't take fat distribution into account. Yet abdominal (visceral) fat is much more dangerous than subcutaneous fat. A waist circumference > 94 cm (man) or > 80 cm (woman) is a better indicator of cardiovascular risk.

When to consult?

A BMI outside the 18.5 – 25 range doesn't necessarily mean there's a problem. But if you are overweight or underweight AND you feel fatigue, shortness of breath, or joint pain, it's time to see a doctor or dietitian.

BMI is a starting point, not a verdict. Combine it with other indicators (waist circumference, body fat, physical performance) for a complete view.

Questions fréquentes

What is the ideal BMI?
The WHO considers a BMI between 18.5 and 25 to be "normal." But the ideal BMI depends on your morphology, age, and muscle mass. A BMI of 22-23 is often cited as optimal for health.
Is BMI reliable for athletes?
No. Muscular athletes will have a high BMI without being overweight. For them, body fat percentage (measurable by impedance or skinfold calipers) is a much more relevant indicator.
Is BMI the same for men and women?
The formula is identical, but the interpretation can vary. Women naturally have a higher body fat percentage. A BMI of 24 doesn't have the same meaning for both sexes.
How often should you recalculate your BMI?
Every 1 to 3 months if you are in a weight loss or gain phase. Otherwise, once or twice a year is enough. BMI changes slowly.

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